1. Field of the invention
The present invention relates to fiber optic transmission.
2. Description of the prior art
The index profile of optical fibers is generally qualified as a function of the shape of the graph of the function relating the refractive index of the fiber to its radius. The distance r of a point from the center of the fiber is conventionally plotted on the abscissa axis with the difference between the refractive index at that point and that of the cladding of the fiber plotted on the ordinate axis. The expressions xe2x80x9csteppedxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d and xe2x80x9ctrianglexe2x80x9d are therefore used in connection with index profiles to refer to graphs which are respectively step-shaped, trapezium-shaped and triangular. The curves are generally representative of the theoretical or set point profile of the fiber but fiber fabrication constraints can produce a significantly different profile.
In new high bit rate wavelength division multiplex transmission networks it is advantageous to control chromatic dispersion, especially for bit rates of 10 Gbit/s per channel and above. The objective is to limit the broadening of pulses by obtaining substantially zero cumulative chromatic dispersion of the link for all wavelength values of the multiplex A cumulative dispersion value of a few hundred ps/nm is generally acceptable. It is also beneficial to avoid zero values of chromatic dispersion, for which non-linear effects are greater, in the vicinity of the wavelengths used in the system. Finally, it is also beneficial to limit the chromatic dispersion slope over the range of the multiplex to prevent or limit distortion between the channels of the multiplex.
Single mode fibers (SMF) with a stepped index profile are conventionally used as line fibers in fiber optic transmission systems. The assignees of the inventors sell under the designation ASMF 200 a stepped index monomode fiber having a chromatic dispersion cancellation wavelength xcex0 from 1 300 nm to 1 320 nm and a chromatic dispersion less than or equal to 3.5 ps/nm/km in a range from 1 285 nm to 1 330 nm and equal to 17 ps/nm.km at 1 550 nm. The chromatic dispersion slope at 1 550 nm is of the order of 0.06 ps/nm2.km.
To compensate the chromatic dispersion and the chromatic dispersion slope of these SMF, which were originally installed to operate at around 1 310 nm and are now to be used as line fibers in a window around 1 550 nm, the document WO 98/04941 describes a dispersion compensating fiber (DCF) having a high negative chromatic dispersion value, typically less than xe2x88x92150 ps/nm.km, at 1 550 nm. To limit non-linear effects, the fiber has an effective mode surface area greater than 30 xcexcm2 at 1 550 nm. Also, in one of the embodiments described in the above document, the chromatic dispersion slope of the fiber can be from xe2x88x925 ps/nm2.km to xe2x88x920.1 ps/nm2.km at 1 550 nm.
These values enable it to compensate the dispersion of a non-zero dispersion shifted fiber (NZ-DSF) whose chromatic dispersion is from 6 ps/nm.km to 10 ps/nm.km and whose chromatic dispersion slope is less than 0.07 ps/nm2.km at 1 550 nm.
DSF have substantially no chromatic dispersion at the transmission wavelength at which they ore used, which is generally different from the wavelength of 1.3 xcexcm at which silica has substantially zero dispersion. In other words, the non-zero chromatic dispersion of the silica is compensated (hence the use of the term xe2x80x9cshiftedxe2x80x9d) by increasing the index difference xcex94n between the core of the fiber and the optical cladding. The index difference shifts the wavelength at which there is substantially zero chromatic dispersion. It is obtained by introducing dopants into the preform during its fabrication, for example by an MCVD process known in the art and not described in detail here.
NZ-DSF have a non-zero chromatic dispersion at the wavelength at which they are used.
The DCF described in the document WO 98/04941, although having optical characteristics such that it could compensate the dispersion of DSF and NZ-DSF, and more particularly fiber having the characteristics mentioned above, is not suitable because it has very high curvature losses (of the order of 0.3 dB/m for a winding of 100 turns with a radius of 30 mm) because of the limited index difference between the central part of its core and its optical cladding.
The object of the present invention is to provide a dispersion compensating fiber capable in particular of compensating the chromatic dispersion of an NZ-DSP whose chromatic dispersion is from 5 ps/nm.km to 11 ps/nm.km and whose chromatic dispersion slope is less than 0.08 ps/nm2.km at 1 550 nm, combined with curvature losses enabling it to be used effectively in current optical transmission systems.
To this end, the present invention proposes a dispersion compensating optical fiber having at a wavelength of 1 550 nm a chromatic dispersion less than or equal to xe2x88x9240 ps/nm.km, a negative chromatic dispersion slope, a ratio of chromatic dispersion to chromatic dispersion slope from 50 nm to 230 nm, an effective surface area greater than or equal to 12 xcexcm2 and curvature losses less than or equal to 0.05 dB for 100 turns wound with a radius of 30 mm.
The fiber preferably has at a wavelength of 1 550 nm a chromatic dispersion greater than or equal to xe2x88x9250 ps/nm.km.
The upper limit of the ratio of chromatic dispersion to chromatic dispersion slope can be chosen as 200 nm, 180 nm or 160 nm.
The lower limit of the ratio of chromatic dispersion to chromatic dispersion slope can be chosen as 80 nm, 100 nm or 120 nm.
All combinations of the above upper and lower limit values can be combined to determine a preferred range for the ratio.
In one embodiment the fiber has at a wavelength of 1 550 nm on effective surface area greater than or equal to 15 xcexcm2 and preferably greater than or equal to 20 xcexcm2.
In another embodiment the fiber has at a wavelength of 1 550 nm an attenuation less than or equal to 1 dB/km.
In a further embodiment the fiber has at a wavelength of 1 550 nm a mode diameter greater than or equal to 4 xcexcm.
The fiber advantageously has at a wavelength of 1 550 nm a sensitivity to microcurvatures less than or equal to 1 and preferably less than or equal to 0.5.
The fiber can have a rectangular index profile with a depleted trench and a ring. In this case the difference between the index of the buried part and the index of the cladding is preferably greater than or equal to xe2x88x928xc3x9710xe2x88x923.
The invention also proposes use of a fiber in accordance with the invention as compensating fiber in a wavelength division multiplex fiber optic transmission system.
The compensation fiber can then be incorporated into a cable and used as line fiber or placed in a compensation module.
The invention then proposes a wavelength division multiplex fiber optic transmission system comprising a first line fiber section and a second line fiber section according to the invention.
The line fiber of the first section advantageously has at a wavelength of 1 550 nm a chromatic dispersion from 5 ps/nm.km to 11 ps/nm.km and a chromatic dispersion slope less than or equal to 0.08 ps/nm2.km.
In one embodiment the ratio of the length of the first section to the length of the second section is substantially the reciprocal of the absolute value of the ratios of the chromatic dispersions at 1 550 nm of the fibers of the first section and the second section.
The cumulative chromatic dispersion for each channel between 1 530 nm and 1 620 nm is advantageously less than 100 ps/nm and preferably less than 50 ps/nm on average over 100 km of transmission.
The invention finally proposes a wavelength division multiplex fiber optic transmission system comprising line fiber and fiber in accordance with the invention as compensation fiber in a compensation module.
The line fiber of the first section advantageously has at a wavelength of 1 550 nm a chromatic dispersion from 5 ps/nm.km to 11 ps/nm.km and a chromatic dispersion slope less than or equal to 0.08 ps/nm2.km.
The ratio of the length of the line fiber to the length of the compensation fiber is preferably substantially the reciprocal of the absolute value of the ratio of the chromatic dispersions at 1 550 nm of the line fiber and the compensation fiber.
The cumulative chromatic dispersion for each channel from 1 530 nm to 1 620 nm is advantageously less than 100 ps/nm and preferably less than 50 ps/nm on average over 100 km of transmission.